玻色子和虚粒子哪个好（新物理真的要来了吗）
玻色子和虚粒子哪个好（新物理真的要来了吗）粒子物理学的标准模型。我们所能认知的宇宙中的一切，都是由无数个微观粒子所组成的，而微观粒子组成更复杂的结构的过程中，不但需要累积“砖块”（也就是实物粒子），还需要将这些“砖块”粘接起来的“水泥”（也就是粒子间的相互作用力）。科学家们描述微观粒子以及它们之间的相互作用的理论叫做粒子物理学的标准模型。北京时间2022年4月8日凌晨（芝加哥当地时间4月7日），费米实验室的CDF国际合作组通过全球多家媒体同步发布了W玻色子质量测量的迄今最精确结果，比粒子物理标准模型的预期值偏高7个标准偏差。该研究成果以封面文章发表在4月8日《科学（Science）》杂志正刊上（图1）。标准模型与W玻色子什么是W玻色子？
新物理乌云再现？
20世纪初，物理学已经发展到了非常完美的程度。但是，经典物理大厦上空飘浮的两朵小小的乌云，却最终发展成为一场推倒大厦的风暴，并促成了相对论和量子力学的建立。
百年之后，2021 年 4 月 7 日上午，美国费米国家加速器实验室（Fermilab）公布了缪子 g-2 实验组对于缪子反常磁矩的首个测量结果，瞬间掀起了人们对于物理学发展的新讨论。整整一年以后的2022年4月7日，费米实验室再次发布了一项新的实验结果，又一次引发了物理界的大讨论：新物理乌云真的要出现了吗？
图1，4月8日《科学》杂志封面新闻
北京时间2022年4月8日凌晨（芝加哥当地时间4月7日），费米实验室的CDF国际合作组通过全球多家媒体同步发布了W玻色子质量测量的迄今最精确结果，比粒子物理标准模型的预期值偏高7个标准偏差。该研究成果以封面文章发表在4月8日《科学（Science）》杂志正刊上（图1）。
标准模型与W玻色子
什么是W玻色子？
我们所能认知的宇宙中的一切，都是由无数个微观粒子所组成的，而微观粒子组成更复杂的结构的过程中，不但需要累积“砖块”（也就是实物粒子），还需要将这些“砖块”粘接起来的“水泥”（也就是粒子间的相互作用力）。科学家们描述微观粒子以及它们之间的相互作用的理论叫做粒子物理学的标准模型。
粒子物理学的标准模型。
外圈的12种粒子是构成物质世界的实物粒子，中圈是传递相互作用力的规范玻色子，而最里面是赋予其它粒子质量的希格斯玻色子。
粒子物理标准模型理论描述了组成所有物质的61种基本粒子，也阐释了它们之间的三种基本相互作用——电磁力、弱力和强力，是物理学最基本的理论之一。根据标准模型，相互作用力是通过基本粒子来传递的，比如带电粒子之间的电磁力是通过光子来传递的，其力程无限远。强力是夸克间通过胶子传递的，弱力则是由W、Z等叫作中间玻色子的粒子来传递的，力程极小（小于 米），而且力很弱，仅为电磁力的万分之一左右。W玻色子正是借用了Weak force（弱力）的首字母来命名的。
图2，标准模型 (Credit: TriTertButoxy/Stannered at English Wikipedia)
W玻色子另一神奇的特征是，不同于传递电磁力的零质量光子，它居然有质量。而且，W的质量直接影响了费米常数，它决定了太阳中心聚变过程的速率，如果这过程太快了，恐怕地球上就没有足够时间演化出人类。
上世纪中叶，格拉肖（Sheldon L. Glashow）、温伯格（Steven Weinberg）和萨拉姆（Abdus Salam）统一了弱力和电磁力，并因此获得了1979年诺贝尔物理学奖。与此同时，实验粒子物理学家们一直希望能在高能实验中寻找到W玻色子，但由于它的质量较重，需要能量足够高的加速器，才容易从复杂的实验数据中观测到踪迹。
这项努力一直延续到1983年，在欧洲核子中心（CERN）的超级质子同步加速器（Super Proton Synchrotron）上，鲁比亚（Carlo Rubbia）和范德梅尔（Simon van der Meer）等人带领UA1和UA2合作组，终于在实验上发现了W玻色子和Z玻色子存在的证据，并于次年获得诺贝尔物理学奖。
图3，CDF探测器， 费米的老加速器Tevatron是LHC之前的最高能加速器（1985-2011）
美国费米实验室的Tevatron曾为世界上最大的对撞机，在Tevatron里，质子和反质子被加速到它们的静止质量的1000倍，然后发生碰撞，从而大量产生W玻色子。
新物理真的要来了吗？
为什么科学家们认为W玻色子质量的偏差暗示着新物理的存在？
在粒子物理标准模型中，W玻色子的质量通过内部对称性和标准模型中的其他参数紧密联系在一起。粒子理论学家可以通过已经测得的希格斯玻色子的质量、Z玻色子的质量、顶夸克的质量、缪子的寿命计算出W玻色子的质量。最新计算给出W玻色子的质量为80357±6 。而CDF合作组的最新测量结果（目前最精确的结果）显示其W质量测量值为80433.5±9.4 （图3中CDF II所展示的结果）。
二者之间存在着7个标准偏差。也就是说在标准模型的预言是正确的情况下，CDF实验观测到这样的实验结果的可能性仅为大概 。如果CDF的最新结果是正确的，那么在标准模型的框架下W玻色子的质量和以前测得的Z玻色子的质量、顶夸克的质量、希格斯玻色子的质量还有缪子的寿命是不相容的。
这意味着粒子物理标准模型并不完备，一些物理因素之前可能被科学家们忽略了，因此需要引进新物理的修正。
图4 ，LHC 的 ATLAS 探测器. (图片源自 CERN)
需要注意的是，从图3我们可以看到，CDF最新的结果和ATLAS的测量结果也存在大约3个标准差的偏差，而ATLAS的结果和标准模型的结果在一个标准差之内是吻合的。因此，标准模型对W玻色子的质量的预言是否有偏差也还需要其他的实验进一步检验。中国参与的大型强子对撞机上的ATLAS实验、CMS实验、LHCb实验正在进行相关的研究。规划中的环形正负电子对撞机（CEPC）、未来环形对撞机（FCC）等，将能够对W玻色子的质量做更为精细的测量，进一步检测标准模型计算是否需要被修改或扩展。
图5，CEPC设计示意图
附外网文章：
Why Scientists Are Worried About the W Boson: 'Something Is Amiss'
And that something could totally change one of the universe's most fundamental frameworks.
为什么科学家担心W玻色子：“出了点问题”
——因为这是可能会彻底改变宇宙最基本框架的东西之一
You've probably heard of protons positive specks anchoring atoms. You've likely come across electrons negative blips roaming around those protons. You may have even pondered photons the stuff coming out of lightbulbs in your room.
But right now we need to worry about an odd little particle that usually escapes the limelight: the W boson.
Along with its partner-in-crime the Z boson the W boson dictates what's called the "weak force." I'm going to save you from the rabbit hole of how the weak force works because it involves physics that'll explode our minds. Trust me. Just know that without the weak force the sun would basically stop burning.
Anyway there's drama with the W boson. According to a paper published Thursday in the journal Science 10 years of unimaginably precise data suggest the particle is more massive than our physics predicts. Unless you're a physicist at first glance this might sound trivial. But it's actually a major problem for…kind of everything.
More specifically it spurs a paradox for the Standard Model of particle physics a well-established evolving theory that explains how all the universe's particles behave -- protons electrons photons and even those we don't really hear about like gluons muons I could go on. The W boson is in there too.
"It is one of the cornerstones of the Standard Model " said Giorgio Chiarelli research director of the Istituto Nazionale di Fisica Nucleare in Italy and co-author of the study.
But here's the crux of the Standard Model. It's like a symbiotic particle world. Think of each particle in the model as a string perfectly organized to tie everything together. If one string is too tight stuff starts getting wonky -- it doesn't matter which string. As such the Standard Model predicts a few parameters for each "string " or particle and a very important one is the W boson mass.
Simply put if this particle doesn't equal that mass the rest of the model wouldn't quite work out. And if that were true we'd have to change the model -- we'd have to change our understanding of how all the particles in the universe work.
Well remember the new paper? We're pretty much entering that worst case scenario.
A decade of calculations measurements cross-checking head-scratching and deep breathing from about 400 international researchers concluded that the W boson is slightly heavier than the Standard Model predicts it should be.
"It's not a big difference but we can really see clearly that it is different " said David Toback a particle physicist from Texas A&M University and co-author of the study. "Something is amiss."
You might be wondering if we're sure about this. The science community had the same reaction which is why researchers are now laser-focused on confirming that this greater W boson mass is really the case.
"It could be we got it wrong " Toback said. But he quickly added "We don't think so."
It's because as Toback explains the team "measured this tiny difference with such incredible precision that it sticks out like a sore thumb." And fascinatingly these measurements sort of resemble crime-scene-style deduction.
Watching for what's missing
To get a W boson in the first place you literally have to smash two protons together.
That produces an array of other Standard Model particles and scientists just have to hope one of those is the one they want to examine. (In this case that's the W boson).
For the new measurements the researchers used collision data from a now-out-of-service particle accelerator at the Fermi National Accelerator Laboratory in Illinois. Thankfully it did make some W bosons and in fact held enough W boson data to procure about four times the amount as used in previous measurements. Jackpot.
But there's a complication. The W boson is fleeting. It rapidly splits into two smaller particles so you can't measure it directly. One of those is either an electron or muon which can be measured directly but the other is arguably even stranger than the W boson itself: A neutrino.
Neutrinos are aptly called "ghost particles " because they don't touch anything. They're even zooming through you right now but you can't tell because they don't touch the atoms that make up your body. Eerie I know.
This ghostly hurdle means scientists have to get creative. Enter the art of deduction.
Once neutrinos vanish they leave behind a sort of hole. "The footprint of the neutrino is missing energy " Chiarelli said. "This tells us where the neutrino went and how much energy was carried away."
It's kind of the same concept as an X-ray. "The X-ray goes through but for the point in which you have some piece of metal you can see the shape " Chiarelli said. The "shape" is the "missing energy."
Then after decoding the neutrino the scientists used a bunch of complex equations to add it up with the electron or muon data. That led to the overall W boson mass. This measurement was done many many times to make sure everything was as precise as possible. Plus all the data was bolstered by theoretical calculations that have matured since the last time the W boson was measured.
Yet… there's another complication.
As with all scientific pursuits there is no right or wrong answer. There is only the answer. But as with all human thought there's the possibility of bias and the team did not want to fall victim to such personal error. Toback quotes Sherlock Holmes to explain the team's mentality: "One must find theories to suit facts and not facts to suit theories."
"Is it more stressful?" he remarked. "Yes but nature doesn't care about my stress. What we want is to know the answer."
Therefore not only did the team double triple quadruple check their data they did so while fully blinded to the final answer. When the box with the W boson mass result was opened everyone would be looking at it for the first time.
Fast-forward to the year 2020 when tensions are high the box is finally opened and the W boson mass is in clear contention with the Standard Model's prediction.
"It was not a Eureka moment " Chiarelli said. "It was a rather sobering moment. We were skeptical. Science is organized in skepticism."
But over time even that skepticism faded and here we are.
This all seems very solid. Now what?
In a sense this information has been a long time coming. "We've known since the beginning that the Standard Model cannot be the ultimate theory " Chiarelli said.
For instance the Standard Model famously cannot explain gravity dark matter and many other elusive aspects of our universe.
One idea is that this new information about the W boson mass might mean we need to add some particles to the Standard Model to account for the change. This could in turn impact what we know about the famous Higgs Boson or "god particle " which was finally detected in 2012 and met with world-shattering applause.
"But we're not there " Toback said. "That would be pure speculation."
And rather than speculate Toback and Chiarelli agree that we just have to follow the facts even if we know the facts will one day lead us to a new fundamental theory of particle physics.
"It's like moving in the dark " Chiarelli said. "You know that there is one way which is correct but you don't know where…maybe our measurement can give us the right direction to move."
参考链接：
https://www.science.org/doi/10.1126/science.abk1781
https://www.science.org/content/article/mass-rare-particle-may-conflict-standard-model-signaling-new-physics
